According to the stipulations which apply for breathing apparatus with a carried gas supply, as a rule air at a pressure of at most 300 bar, there must be provided a warning device which tells the user when a predetermined volume of breathing gas remains at his disposal. This predetermined volume of gas normally corresponds to 20 percent of the maximum filling pressure of the gas store.
Such warning signals are generated, for example, by a whistle which is automatically activated when the pressure in the gas supply has fallen to the predetermined level. Depending upon the type of the breathing apparatus and the gas supply, and upon the stipulations regulations applicable in the user's country, the actuation value of the warning signal may vary between 30 and 75 bar, within which pressure range the signal or warning apparatus should be actuable. In order to obtain the correct aspiration flow to the whistle, gas is supplied through a nozzle which is connected to a pressure regulator.
Such a regulator may consist of a first regulation stage in the breathing conduit of the apparatus, or of a specific pressure regulator which is included in the signal apparatus and is connected with direct gas supply from the gas source.
The gas flow which is consumed for making the signal may, for example according to the CEN standards, not exceed 5 liters per minute on average throughout the entire time when the pressure in the gas supply falls from the actuation value towards zero. With prior-art regulators, there will be obtained a gas volume consumed by the whistle which increases--normally linearly--as the pressure falls in the gas supply from the actuation value towards zero.